1. Field of the Invention
The present invention relates generally to four stroke internal combustion engines, and in particular to the valve timing for such engines.
2. Description of Related Art
It is known to recirculate exhaust gases back into engine combustion chamber(s) to assist in reducing overall exhaust emissions, in particular, NOx emissions.
It has been proposed in relation to two-stroke direct injected engines to recirculate exhaust gases back into a combustion chamber by running a separate conduit externally of the combustion chamber from the exhaust manifold back to the combustion chamber and/or intake manifold. See, for example the applicant's earlier international application no. PCT/AU94/00288. However, this process may, in certain applications, have a number of disadvantages. For example, the system is dependent on the pressure of gas in the exhaust manifold being sufficiently higher than that in the intake manifold to attain the required exhaust gas flow through the conduit. In addition, modifications to the engine are required to install such an exhaust gas recycling system.
During each operating cycle of a typical four stroke internal combustion engine, there is a valve overlap period, where both the inlet valve(s) and the exhaust valve(s) of each cylinder are open at the same time. The overlap period helps in the effective flow of gas through the cylinder.
The valve overlap period typically occurs between the opening of the inlet valve(s) and the closing of the exhaust valve(s), the exhaust valve closing after the inlet valve has opened such that both valves are open during the overlap period. Measured in relation to the crank angle of the engine, the inlet valve typically opens at about 10.degree. Before Top Dead Centre (BTDC) and the exhaust valve typically closes at about 10.degree. After Top Dead Centre (ATDC) in a conventional four stroke engine. The crank angle at which exhaust valve closure occurs is usually quite strictly determined to be in the range of between 5.degree. and 15.degree. ATDC in modern four stroke cycle engines, to ensure efficient operation of the engine.
The point of closing of the exhaust valve, particularly at idle and part load, regulates the quantity of exhaust gases that can flow back into the cylinder through the exhaust valve during the induction stroke of the subsequent combustion cycle. It would therefore be advantageous, under certain engine operating conditions, to delay the closing of the exhaust valve to thereby increase the amount of exhaust gases drawn back into the cylinder through the exhaust valve.
There are however limitations in the extent that the closing of the exhaust valve can be delayed in conventional carburettor or intake manifold fuel injected engines as this can lead to combustion instability in conventional four stroke engines. One reason is that the redrawing of exhaust gas into the cylinder can displace fresh fuel and air charge entrained in the bulk air intake, leading to increased combustion instability. Furthermore, if the valve overlap period in a conventional four stroke internal combustion engine is too long, fresh charge can "short circuit", passing directly from the inlet port to the exhaust port without participating in a combustion event. Clearly, this can result in significant fuel wastage as well as lead to increased HC emissions.
It is an object of the present invention to provide an improved method of controlling the operation of a four stoke internal combustion engine.